1. Field of the Invention
The present invention relates to a gain flattening optical filter, to an optical amplifier comprising such an optical filter and to a method for manufacturing such an optical filter.
2. Technical Background
In this description, reference is made to optical fibres, but this reference shall be intended as a matter of example only and not as a limitation, since the technology described is equally applicable also to integrated optical waveguides.
Typically, the optical fibres used for telecommunications have the core doped with germanium to alter the refractive index. Doping with germanium induces a photosensitivity property to the UV radiation, which can be used to locally modify the refractive index through UV illumination, in such a way as to obtain a Bragg grating in the optical fibre.
As known, an optical fibre Bragg grating is a portion of fibre that has, in its core, an essentially periodic longitudinal modulation of the refractive index. Said structure has the property of back-reflecting the light passing through it in a wavelength band centered around a predetermined wavelength xcexB, known as Bragg wavelength. The Bragg wavelength xcexB is related to the effective refractive index neff and to the grating period xcex9(z) (both typically being function of coordinate z along the fibre axis) by the following Bragg phase-matching condition (see, for example, international patent application WO 99/31537):
xcexB=2neffxcex9(z)xe2x80x83xe2x80x83(1) 
By selectively reflecting a predetermined wavelength band, an optical fibre Bragg diffraction grating may be interposed in an optical fibre transmission line to filter a multi-wavelength optical signal.
The pattern of the refractive index n along axis z of the fibre can be expressed as follows:
n(z)=n0(z)+xcex94n(z)sin(2xcfx80z/xcex9(z))xe2x80x83xe2x80x83(2) 
where n0(z) is the local mean value of the refractive index (hereinafter also referred to as xe2x80x9cmean refractive indexxe2x80x9d) and xcex94n(z) represents the local envelope of the refractive index modulation (shortly referred to as xe2x80x9crefractive index envelopexe2x80x9d, or alternatively as xe2x80x9crefractive index modulation amplitudexe2x80x9d). More precisely, xcex94n(z) defines, for each position z along the fibre, the distance between the upper and the lower envelope lines of the refractive index representative curve. For example, when the upper ad the lower envelope lines are straight lines, their distance and, therefore, the refractive index envelope xcex94n(z), are constant. The effective refractive index neff is proportional to the mean refractive index n0(z) through a term defining the confinement factor (typically indicated with xcex93) of the fundamental mode of the fibre.
A known method for writing periodic refractive index lines in the fibre comprises directing a UV writing beam onto the fibre through a periodic phase-mask facing the fibre, so as to illuminate the fibre with a predetermined UV fringe pattern. The pitch of the lines or fringes of the interference patter projected onto the fibre is half that of (i.e. twice as close as that of) the lines physically present (e.g. etched) in the phase mask. For example, if the phase mask has a xe2x80x9cphysicalxe2x80x9d pitch of 1 xcexcm, the lines projected onto the fibre have a pitch of 0.5 xcexcm.
A different technique for writing periodic refractive index lines makes use of a holographic arrangement for generating an appropriate UV fringe pattern on the fibre lateral surface.
On the basis of the pattern of the refractive index, uniform gratings, so-called xe2x80x9cchirpedxe2x80x9d gratings and apodised gratings are known.
In uniform gratings, the terms n0(z), xcex94n(z) and xcex9(z) are constant. The reflection spectrum of a uniform grating typically exhibits a central peak at the Bragg wavelength, and a plurality of secondary lobes. Said secondary lobes can be disadvantageous in some applications, for example when the Bragg grating is used to filter a channel (centered at a predetermined wavelength) in a multi-channel optical transmission system. In this case, in fact, the secondary lobes of the reflection spectrum introduce an undesired attenuation in the transmission channels adjacent that to be filtered.
In apodised gratings, the term xcex94n(z) is suitably modulated in order to have a reduction of secondary lobes. Such a grating can thus be advantageously used for filtering a channel in a multi-channel system, since it reduces the above-mentioned problem of the attenuation of the channels adjacent that filtered.
In chirped gratings, either of the terms n0(z) and xcex9(z) is variable, and the chirping may be referred to as xe2x80x9camplitude chirpingxe2x80x9d or xe2x80x9cpitch chirpingxe2x80x9d, respectively. Due to the variability of n0(z) or xcex9(z), and due to the fact thatxe2x80x94according to what stated abovexe2x80x94the Bragg wavelength is proportional to the product between n0(z) and xcex9(z), chirped gratings have a relatively broad reflection band. FIGS. 1a, 1b and 1c respectively show the qualitative pattern of the refractive index in the case the term n0(z) is modulated, the qualitative pattern of the refractive index in the case the term xcex9(z) is modulated (for example, with a continuous variation form about 500 nm to about 502 nm), and the typical reflection spectrum of a chirped grating. As it can be noted from FIG. 1c, the reflection spectrum shows a peak that is relatively broad.
Pitch chirping is predominantly used, as it offers broader grating bandwidths and relative ease of production. The chirp can be incorporated in to the fibre during the fabrication process (xe2x80x9cintrinsic chirpxe2x80x9d) or can be obtained by applying an external perturbation to a fibre already including a non-chirped grating (xe2x80x9cextrinsic chirpxe2x80x9d).
Intrinsic chirp can be introduced in different ways, for example by using a non-uniform period phase-mask, by subjecting the filter to strain of temperature gradients during the writing process, by writing gratings on pre-strained fibres or in fibre tapers, by curving the fibre in a standard phase-mask set-up, by tilting the fibre with respect to a phase-mask, or by interfering wavefronts of dissimilar curvatures in a holographic arrangement. These methods of writing broad-bandwidth gratings, which require very good mechanical stability and spatial coherence properties of the writing beam, suffer from the disadvantage of allowing a limited choice of filter spectral response.
To form an extrinsic chirp, external perturbations such as strain gradients or temperature gradients can be used. This external perturbation can also be used to vary the chirp so as to tune the filter spectral response. U.S. Pat. No. 6,169,831, in the name of Lucent Technologies, for example, teaches how to use a temperature gradient or a strain gradient as an extrinsic gradient for this purpose. These devices have, however, the drawback that relatively large external gradients perturbations are required to obtain a suitable range of chirping, and such perturbations may have a negative impact on the reliability of the fibre.
It is known to use chirped gratings for compensating the chromatic dispersion in a WDM transmission system.
WO 98/08120, in the name of PIRELLI CAVI E SISTEMI S.P.A., tackling the problem of chromatic dispersion, proposes a technique (defined xe2x80x9cContinuous Fibre Grating Techniquexe2x80x9d) to produce a fibre grating suitable to compensate said dispersion. According to this technique, a fibre, exposed through a mask to a UV radiation periodically time modulated, is continuously translated along its axis by a translation stage, so that subsequent exposures produce overlapped fringes. Arbitrary phase profiles and in particular a linear chirp can be built up by inducing phase shifts along the grating as it is fabricated.
WO 98/08120 also refers to a previously developed technique, described in U.S. Pat. No. 6,072,926 (Cole et al.), wherein a phase mask is scanned by a writing laser beam to generate the grating pattern. The fibre and the phase mask are moved with respect to one another during the writing process, to vary the grating properties along the length of the grating. Relative movement in a single direction provides a change of grating pitch, and so can be used to fabricate chirped or multi-wavelength gratings. Bi-directional dither alters the strength of the grating, and so can be used to fabricate apodised gratings.
WO 099/31537, in the name of University of Southern California, describes a nonlinearly-chirped fibre grating for achieving tunable dispersion compensation, chirp reduction in directly modulated diode lasers, and optical pulse manipulation. The nonlinearly-chirped fibre grating may be made by a near-UV technology that uses an interference pattern produced by a phase mask, and has a mechanism to adjust the Bragg phase-matching condition. In one embodiment, the grating is made of a mechanically stretchable or compressible material and has a nonlinearly chirped grating period, and a transducer is engaged to the grating to uniformly change the overall length of the grating, thus providing a spectral shift in the operating spectral range. In another embodiment, the grating has a uniform grating period and a nonlinearly chirped effective index of refraction along the grating direction, and the grating material is responsive to a spatially-varying external control field (such as an electric field, an electromagnetic field, or a temperature field) so that the nonlinear chirp can be adjusted to change relative delays of different spectral components. In yet another embodiment, the grating has a nonlinearly chirped grating period and an externally adjustable spatial profile in the effective index of refraction. The overall length and the effective index of refraction of the grating can be individually adjusted to change the relative delays of different spectral components and to shift the operating spectral range of the grating.
A different application of fibre Bragg grating is for gain equalization of optical amplifiers in multi-wavelength transmission systems. For this application, apodised Bragg grating, sometimes chirped, are typically used.
The article of M. Ibsen et al., xe2x80x9cCustom Design of Long Chirped Bragg Gratings: Application to Gain Flattening Filter with Incorporated Dispersion Compensationxe2x80x9d, IEEE Photonics Technology Letters, Vol. 12, No. 5, May 2000, presents and experimentally demonstrates relationships between the refractive index modulation, the chirp-rate or dispersion and the transmission loss through, and reflection of, chirped Bragg gratings, and applies them to the design of a gain flattening filter with incorporated dispersion compensation. In the described example, the grating (which is going to be operated in reflection) is apodised over 10% of the total length at either ends, in order to reduce the ripples in the reflection and dispersion profiles.
U.S. Pat. No. 6,130,973, in the name of Institut National D""Optique, relates to a method and an apparatus to photo-induce a grating in an optical fibre. In a preferred embodiment, a laser beam is deflected by a mirror towards the fibre at an angle of incidence generally perpendicular to the waveguide axis. A phase mask facing the fibre generates an interference pattern, which produces the modulated refractive index change in the fibre. By modulating a galvanometer that controls the orientation of the mirror, the angle of incidence of the beam is dithered. By choosing an appropriate fraction of the exposure time during which the writing beam is dithered for each writing step, any profile of both the intensity of the modulated refractive index change and its average value may be defined independently. U.S. Pat. No. 6,130,973 also describes how to spectrally design a gain-flattening filter to be incorporated in the middle of a two-stages Erbium Doped Fibre Amplifier (EDFA).
The Applicant has tackled the problem of realizing an alternative Bragg grating optical filter for gain equalization of an optical amplifier.
The Applicant has found that, starting from an optical amplifier having a predetermined gain spectrum in a predetermined wavelength band, it is possible to realize an optical waveguide Bragg grating that has a chirp rate that varies in such a way as to have a transmission spectral response suitable to equalize said gain spectrum in said wavelength band, and that has a substantially constant refraction index envelope (i.e. substantially no apodisation). Preferably, also the mean refractive index no(z) is substantially constant.
The Applicant has verified that the optical filter so obtained has axial dimensions that are lower than that of optical filters formed by apodised Bragg grating, and the optical filter can therefore be housed in a smaller a thermal packaging.
The Applicant has also found that a Bragg grating as described above, having a non-linear chirp suitable to reproduce a particular target transmission spectrum, can be realized by scanning a UV beam on a linearly chirped phase mask facing the waveguide while the waveguide is translated at a constant speed, the beam being scanned with a predetermined motion law (in particular with a velocity variable in a continuous way) that is related to the particular target transmission spectrum.
According to an embodiment of the present invention that relates to an optical signal amplification device, comprises an optical amplifier having a wavelength-dependent gain in a predetermined wavelength band and an equalization device optically coupled in series to the optical amplifier. The equalization device having a wavelength-dependent transmission function that substantially equalizes the gain of the optical amplifier in said wavelength band. The equalization device includes a waveguide Bragg grating, preferably a fibre Bragg grating, having a substantially constant refractive index envelope and a chirp rate that varies in such a way as to obtain said wavelength-dependent transmission function.
The optical amplifier may comprise an active fibre coupled in series to the equalization device, or a first and a second active fibre coupled in series to the equalization device; in this second case, the equalization device is preferably interposed between the first and the second active fibre.
According to an embodiment of the present invention an optical transmission system comprises an optical transmitter adapted to generate optical signals, an optical transmission line for transmitting the optical signals, and an optical receiver for receiving the optical signals from the optical transmission line. At least one of said optical transmitter, optical transmission line and optical receiver comprises an optical signal amplification device as previously described.
According to one embodiment of the present invention a gain flattening optical filter comprises an optical waveguide Bragg grating, preferably a fibre Bragg grating, having a substantially constant refraction index envelope and a chirp rate that varies in such a way as to have a transmission spectral response suitable to equalize the gain spectral response of a predetermined optical amplifier in a predetermined wavelength band.
The gain flattening optical fiber is preferably a trasmissive amplitude filter. According to one embodiment of the present invention also a method for manufacturing an optical filter suitable for gain equalization of an optical amplifier in a predetermined wavelength band, comprises the steps of
providing a photosensitive waveguide in a writing position along a z axis;
scanning a UV beam at a first velocity along the z axis through a linearly chirped phase mask facing the photosensitive waveguide, so as to generate UV fringe patterns; and
translating the photosensitive waveguide at a second velocity along the z axis during the step of scanning the UV beam, so as to expose different portions of the photosensitive waveguide to successive UV fringe patterns; and
modulating the UV beam in a periodic way related to the position of the photosensitive waveguide during translation thereof, so as to superimpose successive UV fringe pattern exposures; wherein the step of scanning the UV beam comprises varying the first velocity in such a way as to produce a nonlinearly-chirped Bragg grating having a substantially constant refractive index envelope and a transmission function suitable to equalize the gain spectrum of the optical amplifier in said wavelength band. The second velocity is preferably substantially constant and the step of scanning the UV beam preferably comprises translating a mirror that deflects the UV beam.